<div class="htmlview paragraph">Flame propagation has received much attention, but few results directly applicable to operating conditions have been obtained. The paper describes a method devised for measuring the rate of flame propagation in gaseous mixtures and some experiments made to coordinate the phenomena with the important factors entering into engine operation; it depends upon the fact that bodies at a high temperature ionize the space about them, the bodies being either inert substances or burning gases. Experiments were made which showed that across a spark-gap in an atmosphere of compressed gas, as in an engine cylinder, a potential difference can be maintained which is just below the breakdown potential in the compressed gas before ignition but which is sufficient to arc the gap after ignition has taken place and the flame has supplied ionization. These experiments and the recording of the results photographically are described. The spark chronograph, an instrument designed to replace the oscillograph used in these experiments, is also described and its advantages stated. The results so far obtained prove the fundamental idea that a voltage sufficient to arc a gap ionized by the ignition flame is not large enough to break down the compressed gas. A study of turbulence and knocking, using this method, concludes the paper.</div>